In acid-catalyzed hydrocarbon conversions, the use of catalysts based on various forms of Y zeolite has become widespread. For such purposes, the most desirable forms of Y zeolite are those wherein the original zeolitic sodium ions have been replaced by less basic cations such as hydrogen ions, or polyvalent metal ions such as magnesium or rare earth metal ions. Still another desirable form is sometimes referred to in the literature as the "decationized" form, which it now appears may actually consist of a form in which part of the exchange sites are actually cation deficient and in which another portion is satisfied by hydrogen ions. To product the hydrogen and/or decationized forms, the general procedure is to exchange most or all of original sodium ions with ammonium ions, and the resulting ammonium zeolite is then heated to decompose the zeolitic ammonium ions, first forming a hydrogen zeolite, which may upon further heating be at least partially converted by dehydroxylation to the truly decationized form. The hydrogen form, the decationized form, and the mixed hydrogen-decationized forms will be referred to hereinafter collectively as "metal cation deficient" Y zeolites.
A problem which was encountered at an early stage in the development of Y zeolite catalysts was that of thermal and hydrothermal stability. The metal-cation-deficient zeolites were found to be in general more active than the polyvalent metal forms, but did not display the hydrothermal stability of the latter. "Hydrothermal stability" refers to the ability to maintain crystallinity and surface area upon calcination after exposure to water vapor, or upon exposure to water vapor at high temperatures. Severe hyrothermal conditions are often encountered in hydrocarbon conversion processes, either as a result of inadvertent process upsets, or during oxidative regeneration of the catalysts, or in other ways. For these and other reasons most commercial processes such as cracking or hydrocracking utilize a polyvalent metal-stabilized form of the zeolite, even though some degree of activity is thereby sacrificed.
In recent years however this picture has changed somewhat. Several forms of stabilized metal-caton-deficient Y zeolites appear to be available, or at least described in the literature. A common feature involved in the manufacture of such stabilized zeolites appears to be a calcining step, in which an ammonium form of the zeolite is calcined at a relatively high temperature, usually above about 1000.degree. F., either in the presence or absence of added steam. This calcination appears to bring about in varying degrees a removal of lattice alumina from the anionic crystal structure, with a resultant shrinkage in the unit cell size to values ranging between about 24.2 and 24.5 A. Exemplary disclosures of such procedures can be found in U.S. Pat. Nos. 3,293,192 to Maher et al., 3,449,070 to McDaniel et al., 3,354,077 to Handsford, 3,493,519 to Kerr et al., 3,513,108 to Kerr, and 3,506,400 to Eberly et al. My investigations have shown that while the procedures described in each of the foregoing patents are capable of producing a metal-cation-deficient Y zeolite of improved hydrothermal stability, none of such procedures result in a product having the optimum combination of activity, hydrothermal stability, and ammonia stability of the zeolites produced herein.
It should be noted that ammonia stability is in many cases as important as hydrothermal stability. In cracking and hydrocracking processes, the feedstocks often contain substantial quantities of nitrogen compounds which are largely converted to ammonia. These nitrogen compounds, as well as the ammonia, not only suppress catalytic activity in varying degrees, but in many cases, especially when water is also present, tend to destroy surface area and crystallinity of the best steam-stabilied Y zeolite catalysts of the prior art. Ammonia stability is also essential in catalysts which may, after partial deactivation by metal agglomeration, to be subjected to the aqueous ammonia rejuvenation procedure described in U.S. Pat. No. 3,692,692. The ability of the stabilized zeolites described herein to withstand the combined destructive effects of water and ammonia in contrast to other steam-stabilized Y zeolites has not yet been explained.
Briefly summarizing, the critical steps in the manufacture of the stabilized zeolites of my invention are as follows:
1. The initial sodium Y zeolite is subjected to a preliminary ammonium ion exchange to replace most, but not all, of the zeolitic sodium with ammonium ions. PA1 2. The resulting ammonium-sodium zeolite is then calcined in the presence of steam under controlled conditions of time, temperature and steam partial pressure correlated to reduce the unit cell size of the zeolite to between about 24.40 and 24.64 A. PA1 3. the steam-calcined zeolite is then re-exchanged with ammonium salt to replace at least 25 percent, and preferably at least about 70 percent, of the remaining sodium with ammonium ions.
The chemical and/or physical changes which take place during step (2) of the foregoing procedure appear to be primarily responsible for the unique ammonia stability of the final product. As noted above, the unit cell shrinkage which occurs during this step is believed to involve dealumination of the anionic crystal structure. It now appears however that different types of dealumination can take place during this calcination step, depending upon the sodium content of the zeolite from step (1), the steam partial pressure in step (2), and perhaps other factors. Certain thermally and/or hydrothermally stabilized Y zeolites of the prior art, even though displaying a unit cell constant within the ranges produced in step (2) herein, have been found to display markedly inferior stability to ammonia. The ammonia-stable compositions of this invention hence appear to be the result of achieving the necessary unit cell shrinkage under the proper conditions, principally sufficient steam atmosphere and a sufficient residual sodium content in the zeolite. A given unit cell shrinkage obtained in a substantially dry atmosphere, or in the substantial absence of zeolitic sodium results in much inferior products as compared to a product of the same unit cell constant produced in the presence of steam and zeolite sodium ions. It would hence appear that different types of dealumination take place, perhaps at different sites of the anionic crystal structure, depending upon such factors as hydration and cationic influences.
To illustrate the foregoing, U.S. Pat. No. 3,449,070 to McDaniel et al discloses a thermal stabilization treatment which apparently is carried out under anhydrous conditions to produce a hydrothermally stable product having a unit cell size of from 24.40 to 24.55 A, which is mostly within the range produced herein. However, these products have been found to display poor ammonia stability. Also, U.S. Pat. No. 3,493,519 to Kerr et al. discloses a stabilization treatment wherein an apparently sodium-free ammonium Y zeolite is calcined in the presence of steam. This product likewise, through hydrothermally stable, has been found to display inferior ammonia stability. The "ultrastable" Y zeolite compositions prepared by the method of U.S. Pat. No. 3,293,192 have been found to be completely unstable to ammonia, due apparently to excessive unit cell shrinkage, as well as possible deficiency in zeolitic sodium and/or steam during the final calcination treatment. Finally, U.S. Pat. No. 3,354,077 to Hansford discloses a steam stabilization treatment under conditions of time, temperature, steam pressure and zeolitic sodium content which overlap the critical combination of those conditions required herein, but there is no teaching of an ammonia-stable product, or of the critical combination of conditions required to produce such a product. Hansford likewise fails to disclose the final ion exchange step required herein.
In addition to high stability, the zeolites of this invention also display unusually high catalytic activity for acidcatalyzed reactions such as cracking, hydrocracking, isomerization, etc, as compared to the stabilized zeolites of the prior art. This is believed to be due primarily to the fact that thermal stabilization in the presence of both steam and sodium ions brings about a higher degree of stabilization for a given degree of unit cell shrinkage than is achieved in the absence of steam, or in the absence of sodium ions. The dealumination of the anionic crystal structure which occurs during stabilization, and which apparently brings about unit cell shrinkage, also results in the destruction of catalytically active ion-exchange sites. It is therefore desirable to effect a maximum of stabilization with a minimum of dealumination, or unit cell shrinkage. The process of the present invention achieves this objective to a substantially greater degree than prior art methods.
From the foregoing, it will be apparent that there are critical limitations upon each of the major steps of my process, and that these limitations are cooperatively interrelated with each other in such manner as to product a final product which is not only hydrothermally stable and ammonia stable, but highly active. To the best of my knowledge, none of the stabilization treatments described in the prior art teach methods for achieving this optimum combination of properties.
For purposes of this invention, a hydrothermally stable zeolite is defined as one which retains at least about 70 percent of its crystallinity after rehydration and calcination for one hour at 900.degree. F. An ammonia-stable zeolite is defined as one which retains at least about 60 percent of its crystallinity after being subjected to the ammonia stability test hereinafter described.